This work is aimed at accurate studies of model membrane systems using electron-spin relaxation techniques with nitroxide and vanadyl probes. We shall use WSR lineshape studies as well as ENDOR ELDOR, and spin echoes. In our initial work we are concentrating on hydrated dipalmitoyl phosphatidylcholine bilayers and focus on molecular dynamics in the various phases exhibited by this system, which we shall study as a function of temperature, pressure, and sample orientation. The spin-relaxation analyses are based on the rigorous theories of Freed and coworkers for motional and slow-tumbling effects, which have now been extensively aplied to (termotropic) liquid crystals. While previous ESR studies on model membranes have been interpreted approximately, revealing important properties such as membrane fluidity, segmental motion, polarity, probe location, and rates of rotational and translational diffusion, our studies are refining and extending this appoximate work to reveal, on a microscopic level, types of molecular reorientation, cooperativity at the thermal phase transition, biaxiality, the nature of the molecular forces, effects of local structure, and elastic properties. These studies will be extended to protein-lipid interactions using a well established reconstituted membrane system containing biologically active established reconstituted membrane system containing biologically active Ca 2 plus ATP-ase. A primary objective will be to elucidate the dependence of enzyme activity on the state of molecular motion of the membrane lipids, including the pH control of Ca2 plus binding.